Electro-optical modulator

ABSTRACT

The use in an electro-optical modulator of an electro-optical crystal in the form of a solid-solution compound of formula (NH 4 ) x  Rb 1  - x  H 1  - y  D y  SeO 4 , x and y being concentration coefficients varying from 0 to 1, (crystals of hydrogenated or deuterated rubidium or ammonium selenates) in order to modify the polarization, the phase or the intensity of an incident light beam, using a small control voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electro-optical modulator. Such devices areused for modulating light beams, that is to say for modifying theirpolarization, their phase, their frequency or their intensity, bymodifying the medium through which they propagate, by the action of anelectric field.

By way of application of such modulators, mention may be made of:

wavelength multiplexing, making it possible to create a plurality ofbeams of different wavelength from an incident beam of given wavelength,by frequency modulation,

laser pulse compression by frequency modulation,

laser pulse generation by polarization modulation,

spatial switching by polarization modulation,

optical telecommuniation by intensity modulation.

Electro-optical modulators are made either from bulk materials, ingeneral single crystals constituting the electro-optical medium throughwhich the light beam to be modulated passes, or from thin films orwaveguides. The latter make it possible to obtain modulation in a widefrequency range (up to several gigahertz), which is very useful intelecommuniations in order to increase the data transfer rate. Suchdevices are of complex design and expensive.

Although limited to uses at lower frequencies (from several hertz toseveral hundreds of megahertz), modulators made from bulk crystals arewidely used, in particular for intensity or phase modulation functions.

2. Description of the Related Art

Almost all crystal modulators currently marketed use as anelectro-optical material a crystal of KDP (potassium dihydrogenphosphate) or ADP (ammonium dihydrogen phosphate). The document U.S.Pat. No. 5,157,539 describes an electro-optical modulator using acrystal of KDP. The electro-optical qualities of these materials arefully known. They have the advantage of being materials whose synthesisis fully mastered and inexpensive.

Some publications:

T. Tsukamoto et al., Japanese Journal of Applied Physics, Vol. 24(1985), Supplement 24-3, pages 165-168,

R. Poprawski et al., Ferroelectrics, Vol. 79 (1988), pages 245-248,

A. Waskowska and Z. Cafzpla, Acta Cryst., Vol B38 (1982), pages2017-2020,

disclose different electrical properties of RbHSeO₄ and NH₄ SeO₄, andalso its cyrstals in partly denterated, which properties do not discloseif it is possible to use it in an electro-optical modulator.

Among the materials which are used, mention may also be made ofinorganic ferroelectrics (LiNbO₃, KNbO₃, BaTiO₃), which generally havehigh electro-optical coefficients and refractive indices. However, theirdielectric permittivity is high and their production is laborious andpoorly mastered.

The direction of the electric field applied to the electro-opticalmaterial may be orthogonal to (this case is referred to as transverseconfiguration) or else collinear with (this case is referred to aslongitudinal configuration) the direction of propagation of the lightbeam.

The case of a crystal in the form of a parallelepiped will beconsidered, with L and d the dimensions of the crystal, respectively inthe direction of propagation of the beam to be modulated and in thedirection of the electric field. L is equal to d in the case of alongitudinal configuration.

A dominant factor of a modulator is the half-wave voltage, defined byV.sub.π =(a*d)/(n³ *r*L), with

a the wavelength of the beam,

n the refractive index of the crystal,

r the electro-optical coefficient of the crystal.

If L=d, then V.sub.π =V.sub.π* =a/(n³ *r). This is referred to as thereduced half-wave voltage, which is a factor depending only on thenature of the crystral.

The high-wave voltage V.sub.π is the voltage to be applied to thecrystal in order to cause a phase shift of π radians between thecomponents of the polarization of a light beam passing through themodulator, that is to say the change from a maximum to a minimum in theintensity of the light transmitted through a suitably orientedpolarizer. The modulation efficiency of a light beam depends greatly onV.sub.π. It is important for this voltage V.sub.π to be as small aspossible.

The abovementioned hydrogen-bonded compounds (ADP, KDP) haveelectro-optical coefficients with non-negligible values. However, theyhave some drawbacks and actually require high control voltages.

Thus, for a wavelength a equal to 0.633 microns:

V.sub.π *≈15,000 V for a KDP crystal,

V.sub.π *=10,000 V for an ADP crystal.

In order to limit the value of the electric voltage applied, themodulators comprise crystals of large dimension L and small dimension d.This gives voltages V.sub.π of the order of several hundreds of volts.It nevertheless remains necessary to use a unit which amplifies thevoltage applied. This leads to the electrical control device associatedwith the modulator being large and increases the cost of the system.

Furthermore, the electrical control power is proportional to thepass-band. It will therefore be beneficial to have a material whosehalf-wave voltage and dielectric permittivity are as small as possible.

Finally, the use of large control voltages leads to a risk of damage tothe crystal and consequently a reduction in the life of the modulator.

Furthermore, the birefringence of the crystals used to date dependsgreatly on temperature, which may lead to a shift in the operating pointof the modulator. This makes it necessary to install the material in achamber which is perfectly controlled in terms of temperature and/or tocompensate for the natural birefringence. This compensation may beperformed by inserting into the modulator a second crystal whosedimensions are strictly identical to the first and whose orientation issuch that its natural birefringence exactly cancels that of the firstcrystal.

Connecting the crystals in series leads, further to the productionconstraints leading to an increase in the manufacturing costs of themodulators (the crystals are actually in the form of long thin plateswhich are difficult to produce when manufacturing single crystals ofhigh optical quality), to a number of drawbacks due to the increase inthe length of the crystal to be crossed:

limitation of the passband of the modulator. The modulation frequency isan important parameter in the definition of the specifications of anelectro-optical device. The cut-off frequency of an opticalcommunication device, for example, may actually govern the data transferrate. Moreover, a modulator electro-optical crystal can be modelled tofirst approximation by an RC electrical circuit. The value of thecapacitance of a capacitor is proportional to the surface area of itselectrodes. It will therefore be beneficial to use a small crystal inorder for it to have a small equivalent capacitance.

increase in the losses due to absorption, leading to a reduction in theintensity of the light beam passing through the crystal.

SUMMARY OF THE INVENTION

The inventor has made efforts to produce an electro-optical modulatorusing a single crystal which is inexpensive to synthetize and whichmakes it possible to overcome, as far as possible, the constraintsmentioned above. In the case in point, he proposes the use of a type ofmaterial which has been found to have the following characteristics:

a half-wave voltage value which is generally less than theaforementioned values, making it possible to use a modulator employing alow control power, on the basis of a small crystal,

a very low sensitivity to temperature, leading to the possibility ofproducing a modulator without having to provide temperature compensationmeans, and

permitting operation in the same frequency range as conventionalmodulators.

The invention thus proposes the use in an electro-optical modulator ofan electro-optical crystal in the form of a solid-solution compound offormula (NH₄)_(x) Rb₁ -xH₁ -_(y) D_(y) SeO₄, x and y being concentrationcoefficients varying from 0 to 1, in order to modify the polarization,the phase or the intensity of an incident light beam, using a smallcontrol voltage.

The invention also relates to an electro-optical modulator comprising:

an electro-optical crystal,

a voltage source,

electrodes connected to the voltage source in order to produce anelectric field in the crystal,

characterized in that the crystal is a solid-solution compound offormula (NH₄)_(x) Rb₁ -xH₁ - yD_(y) SeO₄, x and y being concentrationcoefficients varying from 0 to 1.

The type of material envisaged in the invention for producing anelectro-optical modulator has a reduced half-wave voltage V.sub.π *which can be limited to approximately 270 volts, that is to say of theorder of 55 times less than for KDP and almost 37 times less than forADP. The voltage V.sub.π consequently remains small without having touse a crystal of large dimension L. This reduces the absorption lossesin the crystal. The fabrication (and more precisely growth) time of thecrystals is also reduced.

The type of crystal proposed in the invention is furthermore five totwenty times less sensitive to temperature variations than mostmaterials currently used (ADP, KDP).

Finally, it makes it possible to modulate light waves both in thevisible range and in the near infrared. It is therefore possible to usethe modulator in the same range as conventionally used crystals, or in amore extended range, in particular in the infra-red.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and particular features will emerge on reading thefollowing detailed description of an illustrative embodiment of theinvention, given by way of indication and without implying anylimitation, and made with reference to the appended drawings, in which:

FIG. 1 represents a first electro-optical modulator produced inaccordance with the invention,

FIG. 2 represents a second electro-optical modulator produced inaccordance with the invention,

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 presents a first electro-optical modulator produced in accordancewith the invention.

It comprises:

a crystal 1 fashioned in the shape of a parallelepiped,

an L-shaped casing 2 used as a support, which allows a light beam to bemodulated to pass through,

a voltage source, not represented.

The casing includes two plugs, one of which, having the referencenumeral 4, is represented in FIG. 1. These plugs make it possible toconnect two opposite faces (one of which is represented, having thereference numeral 5) of the crystal 1 to the voltage source by means ofelectrodes running over an insulating base 3.

Those faces of the crystal 1 which are connected to the voltage sourcewill, in a conventional fashion, be covered with a metal deposit, forexample of gold or silver, in order to create a uniform electric fieldin the volume of the crystal.

In the example which is described, it will be assumed that a light beamto be modulated penetrates perpendicularly to one of the faces of thecrystal, and that the modulator is used in transverse configuration,that is to say that the electric field created in the crystal issubstantially perpendicular to the direction of propagation of the lightbeam in the crystal.

In a conventional fashion, the crystal is oriented in such a way thatthe beam is normal to a plane formed by two of the crystalline axes ofthe crystal.

For example, the beam will be normal to the plane formed by thecrystalline axes a and b, and will therefore propagate along thecrystalline axis c of the crystal.

According to the invention, the crystal will be a solid-solutioncompound of formula:

(NH₄)_(x) Rb₁ -_(x) H₁ -_(y) D_(y) SeO₄, x and y being concentrationcoefficients varying from 0 to 1.

It will therefore be a solid solution of hydrogenated and/or deuteratedammonium and/or rubidium selenate.

A larger concentration of ammonium makes it possible to widen thepassband of the modulator.

A larger concentration of rubidium makes it possible to reduce the valueof the half-wave voltage.

A larger concentration of deuterium makes it possible to widen theoperating spectrum in the infrared, but makes the synthesis of thecrystal more complex. Amongst other things, this requires the use ofheavy water, which is not the case if the crystal is only hydrogenated,that is to say if y=0.

For example, it is possible to choose a crystal of rubidium hydrogenselenate, of formula RbHSeO₄ (x=0, y =0) or ammonium hydrogen selenate,of formula NH₄ HSeO₄ (x=1, y=0).

The voltage source will produce an AC voltage. It will thus be possibleto produce a voltage including an AC component and a DC component. Thepresence of a DC component is one way of compensating for thetemperature drift of the modulator by shifting its operating point. Inpractice, it will not be necessary to produce a DC component because ofthe low temperature sensitivity of the type of crystal employed.

Depending on the nature of the crystal, and more particularly dependingon the value of x, an amplifier will optionally be interposed betweenthe voltage source and the crystal. Thus, if the crystal is formed byrubidium hydrogen selenate, it has a reduced half-wave voltage of theorder of 270 volts. For a crystal having a dimension L of 10 millimetresand a dimension d of 2 millimetres, a half-wave voltage of the order of50 volts will be obtained. It will therefore be possible to omit theamplifier. If the crystal is formed by ammonium hydrogen selenate, thereduced half-wave voltage is approximately ten times higher. Anamplifier for the voltage produced will then preferably be interposed,thus avoiding the necessity of substantially increasing the ratio L/d ofthe crystal, which would increase the absorption losses in the crystal.In practice, it will be possible to produce a modulator whose controlvoltage is limited to approximately 200 volts, that is to say much lessthan the voltages conventionally used.

In a preferred version, the electric field will be directedsubstantially parallel to the ferroelectric axis of the crystal. Whenthe crystal is formed from rubidium or ammonium hydrogen selenate, thiswill be the crystalline axis b. This will make it possible to benefitfrom the reduced half-wave voltage V.sub.π * having the smallest value.This is advantageous in so far as it will be possible to choose acrystal with smaller ratio L/d, for equal control voltage, compared to adifferent orientation of the electric field created. This makes themodulator more compact.

FIG. 2 presents a second electro-optical modulator produced inaccordance with the invention.

In addition to the elements represented in FIG. 1, it comprises:

a polarizer 6, placed upstream of the crystal, the axis of which is at45° to the crystalline axes of the crystal 1 forming the input face ofthis crystal (that is to say the axes a and b in the example described),and in the plane of these axes. The polarizer 6 is thereforeconventionally positioned in the incidence plane of the beam to bemodulated. Its presence makes it possible to use the modulator as aphase modulator or as a polarization modulator.

a second polarizer 6' the axis of which is at 90° to the first, may beplaced downstream of the crystal 1 in order to use the device as anintensity modulator.

a quarter-wave plate 7, the fast and slow axes of which are oriented at45° to the crystalline axes a and b, and in the same plane, may beinserted downstream of the crystal 1, between the latter and the secondpolarizer 6', in order to obtain a linear response of the intensityvariation as a function of the phase shift introduced by theelectro-optical effect.

Of course, the presence of one or more polarizers and of a quarter-waveplate will depend on the use of the modulator, depending on whether thedesire is to modify the intensity of the light beam, its phase, itspolarization, its frequency, or more than one of these characteristicssimultaneously.

The description of the modulator which has just been given is, ofcourse, not limiting.

The crystal may be orientated in a different way. Thus, the electricfield and the direction of propagation of the light wave may be orienteddifferently with respect to the crystalline axes, or else not parallelto the crystalline axes of the crystal.

Although a transverse configuration of the modulator has been described,a longitudinal configuration of the field may, of course, be employed.In contrast to the transverse configuration, the longitudinalconfiguration does not make it possible to reduce the control voltage byaltering the dimensions of the crystal. The longitudinal configurationis more suitable for processing large-diameter light beams requiringcompactness. It can be used for the generation of laser pulses. However,it requires higher electric voltages, and the modulation is thereforetechnologically limited to low frequencies. Furthermore, it requirestransparent electrodes to be fitted. For its part, the transverseconfiguration allows large passbands since the half-wave voltage can bereduced by increasing the propagation distance in the crystal and byreducing the inter-electrode distance. It is, however, more sensitive totemperature variations.

A plurality of crystals may also be arranged in series along thedirection of propagation of the light beam, and oriented in such a waythat their natural birefringences compensate each another.

We claim:
 1. An electro-optical modulator comprising 1) anelectro-optical crystal wherein the electro-optical crystal is in theform of solid-solution compound of the formula: (NH₄)_(x) Rb₁ -_(x) H₁-_(y) D_(y) SeO₄, x and y being concentration coefficients varying from0 to 1, and 2) means for supplying a small control voltage to saidcrystal in order to modify the polarization, the phase or the intensityof an incident light beam.
 2. An electro-optical modulator according toclaim 1, wherein the reduced half-wave voltage is less than around 270V.
 3. An electro-optical modulator according to claim 1, wherein thecrystal used is a solid solution of rubidium hydrogen selenate.
 4. Anelectro-optical modulator according to claim 1, wherein the crystal usedis a solid solution of ammonium hydrogen selenate.
 5. Electro-opticalmodulator comprising:an electro-optical crystal, a voltage source,electrodes connected to the voltage source in order to produce anelectric field in the crystal, wherein the crystal is a solid-solutioncompound of the formula: (NH₄)_(x) Rb₁ -_(x) H₁ -_(y) D_(y) SeO₄, x andy being concentration coefficients varying from 0 to
 1. 6. Modulatoraccording to claim 5, wherein the electrodes are arranged so as toproduce in the crystal an electric field substantially parallel to theferroelectric axis of the crystal.
 7. Modulator according to claim 5,wherein the electrodes are arranged so as to produce in the crystal anelectric field substantially parallel to the crystalline axis b of thecrystal.
 8. Modulator according to claim 5, further comprising apolarizer (6) placed upstream of the crystal.
 9. Modulator according toclaim 5, further comprising a polarizer (6') placed downstream of thecrystal.
 10. Modulator according to claim 5, further comprising aquarter-wave plate (7) placed downstream of the crystal.
 11. Modulatoraccording to claim 5, further comprising two crystals placed in seriessuch that their natural birefringences compensate each other.
 12. Amethod of modifying the intensity of a light beam which utilizes themodulator according to claim
 5. 13. A method of modifying the phase of alight beam which utilizes the modulator according to claim
 5. 14. Amethod of modifying the polarization of a light beam which utilizes themodulator according to claim
 5. 15. A method of modifying the frequencyof a light beam which utilizes the modulator according to claim
 5. 16. Amethod comprising:providing an electro-optical crystal formed from asolid-solution compound having the formula: (NH₄)_(x) Rb_(1-x) H_(1-y)D_(y) SeO₄, where x and y are concentration coefficients each having avalue of either 0 or 1, transmitting a light beam through said crystal,and supplying a control voltage to said crystal as the light beam ispassing through said crystal so as to modulate the polarization, phase,or intensity of the light beam.